Factors representing the quality of synthetic quartz glass substrates include defect size, defect density, flatness, surface roughness (RMS or root mean square), material photo-chemical stability, and surface chemical stability. The quality requirement for defects on substrates becomes severer with the current trend toward further miniaturization of ICs. Accordingly efforts are made to ameliorate the method for manufacturing synthetic quartz glass substrates. In fact, improvements in RMS and defect quality have been introduced. The polishing process generally involves the steps of lapping a substrate to a rough surface, pre-polishing the rough surface to a mirror finish, and precision polishing. The pre-polishing step commonly uses a cerium oxide-based polishing slurry as disclosed in Patent Document 1. The subsequent precision polishing step is typically performed by a float polishing technique. Patent Document 2 discloses the float polishing technique using a polishing slurry containing fine grains of zirconium oxide or aluminum oxide.
However, the ceria-based polishing slurry of Patent Document 1 is uneconomical because of the rising price of cerium. It is urgently required to seek for a replacement. On the other hand, the method of Patent Document 2 using zirconia or alumina suffers from low productivity because a long time is necessary until a rough surface substrate as lapped is polished to a mirror finish.